The invention relates to a picture display device comprising an evacuated envelope, a first side of which is provided with an electroluminescent display screen, an opposite side is provided with an electron gun and in which deflection means are arranged between the electron gun and the display screen with which means, at least during operation, an electron beam generated by the electron gun can be deflected, the electron gun having a portion generating at least an electron beam and being provided with a main lens system having a first electrode, a final electrode and at least one intermediate electrode across which a main lens voltage is gradually applied step-wise during operation so as to form an electron-optical main focusing lens. The invention also relates to an electron gun for use in such a device.
Such a device is known from U.S. Pat. No. 3,932,786. The electron gun described in this Patent comprises six intermediate electrodes between the first electrode and the final electrode of the main lens system and, in comparison with other, more conventional electron guns, it comprises a relatively large number of electrodes. For this reason such a main lens is commonly referred to as DML (Distributed Main Lens), MSFL (Multi-Stage Focus Lens) or MEL (Multi-Element Lens). The separate electrodes of the main lens system in the known device are interconnected by means of a resistive voltage divider so that the main lens voltage is gradually distributed step-wise across the electrodes during operation in order to reduce the magnitude of potential jumps in the main lens system. This leads to considerably improved lens properties as compared with more conventional guns in which the main lens voltage is entirely applied across only two electrodes. Notably spherical aberrations can be adequately suppressed to relatively large electron beam currents without an increase of the lens diameter being required.
Although spherical aberrations can be reduced to an acceptable level in a picture display device of the type described above, spot errors may nevertheless occur due to dynamic focusing errors. Such spot errors arise, inter alia because the path length of the electron beam varies, dependent on the position of the spot on the display screen. This becomes notably manifest with pixels which are further remote from the centre of the display screen.